Fixes #378 - Change the color of growl notification when a test is slow/medium

To allow to spot from the growl notification when a test is slow, change the color of the check mark. The slowest test is used for the color:

* If all tests are fast, check mark is green
* If at least one test is medium and none is slow, check mark is yellow
* If at least one test is slow, check mark is red

The corresponding images have been added.

Authors: Brendan Nee & myself